soil mechanics module, ced 201

7/25/2019 Soil Mechanics Module, CED 201

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MODULE OF SOIL MECHANICS

COURSE CONTENT

Section1. INTRODUCTION: Definition ofsoil, soil mechanics, soil engineering, importance of soil

mechanics in the design of civil engineering structures.Section2. ENGINEERING PROPERTIES OF SOIL:Types of soil, transported & residual soil,

engineering properties of soil, 3-phase diagram for soil and label in terms of volumes and weights,

expressions for various soil properties with problems.

Section3. CONSISTENCY OF SOIL: Liuid limit, plastic limit, shrin!age limit, plasticity index,

flow index, toughness index, shrin!age ratio, linear shrin!age and volumetric shrin!age and its

relationships, liuid limit graph from the given data, use of plasticity chart to classify soils.

Section4. SOIL CLASSIFICATION: "tandard "oil #lassification "ystem, salient features of particle

si$e classification and textural classification, grain si$e distribution curve, significance of D%, D3,

and D' from grain si$e distribution curve, coarse grained and fine-grained soils, co-efficient of

curvature and co-efficient of uniformity and their values for both coarse and fine-grained soils.

Section5. SOIL COMPACTION:#ompaction, proctor compaction test, optimum moisture content

and maximum dry density curve, factors affecting compaction, light and heavy compaction, methods of

field measurements of soil compaction, dry and wet side of the optimum moisture content curve, range

of moisture content corresponding to a dry density eual to ()* of the optimum dry density.

Section. S!EAR STRENGT! OF SOIL: "hear strength, causes of shear stress, factors affecting

shear strength of soil, +ohr-#oulombs failure theory, measurement of shear strength by direct shear

test, unconfined compressive strength test, vane shear test with problems.

Section". EFFECTI#E STRESSES:Total stress, neutral stress, and effective stress, effective stress

formula for submerged soil, saturated soil mass with surcharge, saturated soil mass capillary fringe

with problems.

Section$. PERMEA%ILITY OF SOIL: Different modes of occurrence of soil-water, permeability of

soil, Darcys law and its assumptions, factors affecting the permeability of soil, laboratory methods of

determining the co-efficient of permeability of soils constant head and variable head, expressions for

euivalent permeability of stratified soil for hori$ontal and vertical flow of water, methods adopted for

drainage and dewatering of soils.

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Section&. CONSOLIDATION OF SOIL:"ignificance of consolidation, Ter$aghis theory of one

dimensional consolidation, total and differential settlement, different methods of reducing total and

differential settlement.

P'(ctic() Content/erform laboratory tests on determination of enginering properties of soils0 water content, sieve

analysis0 1tterberg limits, "pecific gravity0 direct shear and triaxial compression test0 /roctor

compaction test etc.

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SECTION 1: INTRODUCTION

1. INTRODUCTION

The term "oil has various meanings, depending upon the general field in which it is being considered.

To a /edologist, "oil is the substance existing on the earths surface, which grows and develops

plant life.

To a 4eologist, "oil is the material in the relative thin surface $one within which roots occur,

and all the rest of the crust is grouped under the term 56#7 irrespective of its hardness.

To an 8ngineer, "oil is the un-aggregated or un-cemented deposits of mineral and9or organic

particles or fragments covering large portion of the earths crust.

"oil +echanics is one of the youngest disciplines of #ivil 8ngineering involving the study of soil, its

behavior and application as an engineering material.

Acco'*in+ to Te',(+-i 1&4$/: "Soil Mechanics is the application of laws of mechanics and

hyda!lics to enineein po#lems dealin with sediments and othe !nconsolidated acc!m!lations

of solid paticles pod!ced #y the mechanical and chemical disinteation of oc$s eadless of

whethe o not they contain an admi%t!e of oanic constit!ent&"

4eotechnical 8ngineering is a broader term for "oil +echanics which mainly deals with soils

mechanics, behavior and reaction to applied external forces.

4eotechnical 8ngineering section of civil engineering studies contains:

"oil +echanics "oil /roperties and ;ehavior and 4eomechanics or advanced soil

mechanics which is mainly a 3D perception of stress distribution in soils.

"oil Dynamics Dynamic /roperties of "oils, 8arthua!e 8ngineering, +achine


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4round =mprovement "urface compaction, deep compaction, dynamic compaction, vibro

- compaction, stone columns and pile compaction for shortening the list.

>nderground 8ngineering and #onstruction Tunneling principles and techniues,

+ethods of tunneling, +ining principles and techniues, 4round geoha$ards and +ining

subsidence, Dewatering and ?aste management.

8ngineering geology 5oc! formation and development, landslides and ground

movements, 5oc! +echanics and 5oc! Testing, 4eoha$ards and +ining related

geoga$ards

"lope "tability and 5etaining "tructures 0- t-e nee* to t* )(n*)i*e, Types of

landslides, =nter-related topics,


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2. SOIL FORMATION AND COMPOSITION

2.1. FORMATION

The definition of soil that is used by a civil engineer is rather arbitrary and is somewhat different from

that used by a geologist, soil scientist or a lay person. =n brief, a civil engineer considers soil to include

all the material, organic and inorganic, overlying bedroc!. owever, the engineer must !eep in mind

that there are many basic definitions and terminologies used to classify and describe both physical and

chemical behavior of soil. ;roadly soils are classified as organic and inorganic.

6rganic soils are mixtures in which a significant part is derived from growth and decay of plant life and

in some cases from the accumulation of s!eletons or shells of small organisms. =norganic soils are

derived from either chemical or mechanical weathering of roc!s. =norganic soil that is still located at

the place where it was formed is referred as residual soil. =f the soil has been moved to the other

location by gravity, water, or wind, it is referred to as transported soil. =n general:

"oil material is the product of roc!

The geological process that produces soil is ?81T85=@4 #hemical and /hysical.

Eariation in /article si$e and shape depends on:

- ?eathering /rocess

- Transportation /rocess by water, wind, glaciersF

Eariation in "oil "tructure Depends on "oil +inerals and Deposition /rocess

So9e oi) te


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Loose sands present also problems in high seismic ris! areas, because seismic loading can

cause liuefaction if the sand is saturated, as well as cause significant settlement.

Loess is a deposit of relatively uniform, windblown silt. =t has relatively high vertical

permeability but low hori$ontal permeability. Eery compressible when saturated. Thus, special

measures for design of hydraulic structures canals, dams.

@ormally consolidated clays are clay soils that have never been subBected to a pressure greater

than the existing pressure. 4enerally high compressible, low ultimate bearing capacity, very

low permeability as other clay soils.

6verconsolidated clay soils are clays that have been subBected to a pressure greater than the

existing one. ighly overconsolidated clays generally tend to have a rather greater ultimate

bearing capacity and are relatively incompressible.

8xpansive soils +ontmorillonite, bentonite, illite, vermiculite are highly plastic clays

resulting from the decomposition of volcanic ash. +ontmorillonite and bentonite swell

considerably when saturated or simply due to increase in moisture content and shrin! due to

decrease in water content. This causes problems in the performance of foundations, sidewal!s,

concrete slabs, and other structural elements if the soil is subBected to seasonal changes in

moisture content. ;entonite is often used as an impermeable barrier or pond liner.

/eat is fibrous, partly decomposed organic matter or a soil containing large amounts of fibrous

organic matter. /eats have a very high value of void ratio and are extremely compressible. "u

and /rysoc! %(G2 reported that the settlement of an emban!ment 2.'H m high and underlain

by H.2C m of peat and %2.C m of peaty clay was 2.%3 m in %3 years. The ultimate settlement of

the emban!ment was predicted to be 2.)( m after 2) years.

2.2. COMPOSITION

The soil is composed by "olids singly or in combination with ?ater and 1ir. "oil is a three phase

material which consists of solid particles that ma!e up the soil s!eleton and voids which may be full ofwater if the soil is IsaturatedJ, may be without water if the soil is IdryJ, or may be Ipartially saturatedJ

if the three elements are represented.

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- Dry

S(t'(te* F))

S(t'(te* P('ti())

H

So)i*

Ai'

Mine'() S;e)eton D' Soi)

?ater

"olid

Mineral Skeleton Fully Saturated


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SECTION 2: ENGINEERING PROPERTIES OF SOIL

(

So)i*

Ai'

0(te'

Mineral Skeleton Partly Saturated Soils


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/hysical and index or classification properties of soils

8ngineering properties of soils refer to those behaviors of a soil reflected by parameters which indicate

the type and conditions of the soil, and provide a relationship to structural properties such as strength,

compressibility, permeability, swelling potential. They are the ones that #ontrol its 8ngineering

;ehavior of soils.

3.1 P!YSICAL PROPERTIES

3.1.1 Seciic G'(7it

The "pecific 4ravity, 4, is the most freuently used uantity and is defined by

GDensity of Material

Density of Water w= =

GUnitWeight of Material

Unit Weight of Water w= =

=t is often found that the specific gravity of the materials ma!ing up the soil particles are close to the

value for uart$, that is 4s 2.')


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>sing volumes is not very convenient in most calculations. 1n alternative measure that is used is the

voids ratio, e. This is defined as the ratio of the volume of voids, Evto the volume of solids, Es, that is

eV

V

v

s

=

where Ev EwMEa

E EaM EwM Es

1 related uantity is the porosity, n, which is defined as ratio of the volume of voids to the total

volume.

n V

V

v=

The relation between e and n can be determined by noting that

Es E - Ev % - n E

@ow,

e V

V

V

n V

n

n

v

s

v= =

= .% %

and hence n e

e=

+%

/orosity in soils varies between .3 sands, silts to .C) clays to .G peat, and is largely determined

by the soil bul! density.

3.1.3 De+'ee o S(t'(tion

The degree of saturation, ", has an important influence on the soil behaviour. =t is defined as the ratio of

the volume of water to the volume of voids.

S=

The distribution of the volume phases may be expressed in terms of e and ", and by !nowing the unit

weight of water and the specific gravity of the particles, the distributions by weight may also be

determined as indicated in Table 2.

S V

V

V

eV

w

v

w

s

= =

%%


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Ew e " Es

Ea Ev - Ew e Es% - "

Table 2 Distribution by Eolume, +ass and ?eight in "oilP-(e #o)9e M( 0ei+-t

1ir e % - "r

?ater e "r e "r Nw e "r Nw"olid % 4sNw 4sNw

@ote that Table 2 assumes a solid volume Es % m3. 1ll terms in the table should be multiplied by Esif

this is not the case.

3.1.4 Unit 0ei+-t

"everal unit weights are used in "oil +echanics. These are the bul!, saturated, dry, and submerged unit

weights. The bul! unit weight is simply defined as the weight per unit volume

bulkW

V=

?hen all the voids are filled with water the bul! unit weight is identical to the saturated unit weight,

Nsat, and when all the voids are filled with air the bul! unit weight is identical with the dry unit weight,

Ndry.


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The submerged unit weight, O, is sometimes useful when the soil is saturated, and is given by

NO Nsat - Nw

3.1.5 Moit'e content

The moisture content, m, is a very useful uantity because it is simple to measure. =t is defined as theratio of the weight of water to the weight of solid material

m W

W

w

s

=

=f we express the weights in terms of e, ", 4sand Nwas before weobtain

?w NwEw Nwe "r Es

?s NsEs Nw4sEs

and hence:

m e S

Gs

=

@ote that if the soil is saturated "% the voids ratio can be simply determined from the moisture

content.

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of )) g. These trimmings are then oven dried and found to have a mass of C) g. Determine the phase

distributions, void ratio, degree of saturation and relevant unit weights.

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"oil materials finer than .G) mm -2 material are analy$ed by means of sedimentation or

hydrometer analysis:

To determine the grain si$e distribution of material passing the G)m sieve the hydrometer method is

commonly used. The soil is mixed with water and a dispersing agent, stirred vigorously, and allowed to

settle to the bottom of a measuring cylinder. 1s the soil particles settle out of suspension the specific

gravity of the mixture reduces. 1n hydrometer is used to record the variation of specific gravity with

time. ;y ma!ing use of "to!es Law, which relates the velocity of a free falling sphere to its diameter,

the test data is reduced to provide particle diameters and the * by weight of the sample finer than a

particular particle si$e.

Fi+'e A c-e9(tic 7ie@ o t-e -*'o9ete' tet

1.5 G'(*in+ c'7e

The results from the particle si$e determination tests are plotted as grading curves. These show the

particle si$e plotted against the percentage of the sample by weight that is finer than that si$e. The

%)

%H

.-2 Lsw GGDv

=


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results are presented on a semi-logarithmic plot as shown in niformity #oefficient #u, #u is also called a$en #oefficient

%'

&.&&&% &.&&% &.&% &.% % %& %&&

&

2&

C&

'&

H&

%&&

/article si$e -mm

*

- line gives the uppermost bound of

soils in terms of /= and LL. olt$ and 7ovacs %(H% suggested careful review and chec! of clay soil

laboratory wor! with /= and LL results plotting on the left side of >- line.

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The mathematical expressions for the two lines are such that 1 Q line corresponds

while >- line corresponds to . The 1- line distinguishes between organic above

and inorganic below clays while the >-line is the possible upper bound of soils once relating the /= to

the LL.

The plasticity chart shows that only smectite and illite groups of clay minerals are expansive.

Depending on their /= and LL values, smectite plots exactly below and parallel to the > Q line whereas

illite plots immediately above and parallel to the 1 Q line olt$ and 7ovacs, %(H%. The non-expansive

clay minerals include !aolinite that plots below and parallel to the 1 Q line. alloysite and chlorite plot

far away below the 1 Q line. 4enerally, expansive clays have wide range of /= while non-expansive

clays have very low /= values as shown on the figure 2.%%.2.% below #asagrande, %(CH0 olt$ and

7ovacs, %(H% and +itchell, %((3 and "iva!ugan, 2%.

#asagrandes /=- /L chart and clay mineral groups olt$ and 7ovacs, %(H%

=n general, smectite group montmorillonite and bentonite has higher consistency limits and plot below

but parallel to >-line. =llite group plts above but parallel to 1-line. The two groups are !nown as

expansive clays which swell and shrin! due to change in moisture content. 7aolin plots below and

parallel to 1-line. 6ther groups such as chlorite and halloysite plot also below 1-line but not parallel to

this one. "ome index properties of important clay minerals are given belw.

2%


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SECTION 4: SOIL CLASSIFICATION

SOIL CLASSIFICATION %ASICS

These lecture notes are based upon ;")(3:%((( and ;"%3GG:%(( but, where appropriate, reference

is made to I8urocode G related documentsJ. These are namely ;" 8@ ="6 %C'HH-%:22, ;" 8@ ="6%C'HH-2:2C and ;" 8@ ="6 %C'H(-%:23. During this transitional stage as the full

recommendations of the 8urocode are being implemented during 2% students are advised to be aware

that published text boo!s are li!ely to ma!e little reference to the 8urocode G ie. 8@ %((G and there

are some maBor differences in the way that soils are described. "tudents should be aware that the final

@ational 1nnex to ;" 8@ %((G was published on 3%st December 2( and that sections of ;")(3 are

currently being re-written to comply fully with the 8urocode.

P'oe o oi) c)(iic(tion

%. /rovides a concise and systematic method for designating various types of soil.

2. 8nables useful engineering conclusions to be made about soil properties.

3. /rovides a common language for the transmission of information.

C. /ermits the precise presentation of boring records and test results.

O6Bect o oi) c)(iic(tion

=s to provide a soil @1+8 and symbol, e.g. 451E8L is 4, based on the results of simple and uic! to

perform therefore economic !ey tests0

%. /article si$e distribution /.".D. or sieve analysis.

2. /lastic properties:

Liuid limit test

/lastic limit test

"oil is initially classified into either coarse or fine soil on the basis of particle si$e.

Co('e oi) G'(n)('/:/hysical characteristics and appearance are influenced by the distribution of

particle si$es within the soil, i.e.R.'3mm %9%'mm. 1 granular soil is classified according to its

/article "i$e Distribution.

Fine oi) Co-ei7e/: /hysical characteristics and appearance influenced by cohesion and plastic

properties plasticity associated with mineral composition and water content. The fine soil is sub-

grouped according to its plasticity. The soil classification is commonly based on grain si$e and soil

consistency. "everal classification systems exist:

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%. >nified "oil #lassification "ystem >"#" 1"T+ D2CHG-%%.

2. 1merican 1ssociation of "tate ighway and Transportation 6fficials 11"T6 1"T+D32H2-(

3. >.". Department of 1griculture >"D1.

C. ;urmister "oil =dentification "ystem). +assachusetts =nstitute of Technology +=T.

4.1 %'o(* C)(iic(tion

4.1.1C)(iic(tion o co('e oi)

These include sands, gravels and larger particles.


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The results are processed and plotted on a semi-log chart of cumulative percentage passing y-axis

verses log of particle si$e in mm x-axis.

G'(*in+:

The shape of the /article "i$e Distribution curve indicates the range of particle si$es within a soil.

#oarse soils are sub-grouped on whether a soil is well graded or"oorly graded.1 well graded symbol ? soil has approximately eual proportions of particles si$es and the curve is

usually smooth. @ote the Till is a well graded soil and the well graded gravel 4? in the /"D chart

below.

1"oorly graded symbol / soil may contain a high proportion of material within a limited particle

si$e band or bands. /oorly graded soil may be further sub-divided into uniform soil and gap graded

soil:

1 poorly graded soil /u, uniform or closely graded has a maBor proportion of the particles lying

between narrow si$e limits. 1s shown by the 8stuary "and, "/u


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Soi) *ec'ition:


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"oil #ompositions:

"oil 1: 4ravelF..F* "andF..F*


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4.1.2 C)(iic(tion o Fine+'(ine* oi)

These include the silts and clays and have particles smaller than ' m.

"ilts: these can be visually differentiated from clays because they exhibit the property of dilatancy.

=f a moist sample is sha!en in the hand water will appear on the surface. =f the sample is then

suee$ed in the fingers the water will disappear. Their gritty feel can also identify silts.

#lays: exhibit plasticity, they may be readily remoulded when moist, and if left to dry can attain

high strengths. The precise boundaries between different soil types are somewhat arbitrary, but the

following scale is now in use worldwide.

G'(7e) S(n* Si)t C)(

C M F C M F # + < # + se the prefix from above with first one

of ? or / and then with one of + or #.

=f ? or / are reuired for the suffix then #uand #cmust be evaluated

C DD

u = '&

%&

C D

D Dc =

3&

2

'& %& .

=f prefix is G then suffix is 0 if #uR C and #cis between % and 3 otherwise use P

=f prefix is S then suffix is 0 if #uR ' and #cis between % and otherwise use P

=f + or # are reuired they have to be determined from the procedure used for fine grained materials

discussed below. @ote that + stands for "ilt and # for #lay. This is determined from whether the soil

lies above or below the 1-line in the plasticity chart shown in


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& %& 2& 3& C& )& '& G& H& (& %&&Liuid limit

&

%&

2&

3&

C&

)&

'&

/lasticity

index

#D

6D

or

+D

#L6L

+Lor

#L

+L

#omparing soils at eual liuid limit

Toughness and dry strength increase

with increasing plasticity index

Fi+'e & P)(ticit c-('t o' )(6o'(to' c)(iic(tion o ine +'(ine* oi)

The final stage of the classification is to give a description of the soil to go with the 2-symbol class. nified #lassification /rocedure is given in


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32


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33


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3C


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E


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3'


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3G


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3H


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3(


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C


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C%


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E"#" classification.

1tterberg limits: Liuid limit LL 32, /lastic Limit, /L 2'

"tep %: Determine the * fines from the grading curve

*fines * finer than G) m %%* - #oarse grained, Dual symbols reuired

C2

&.&&&% &.&&% &.&% &.% % %& %&&

&

2&

C&

'&

H&

%&&

/article si$e -mm

*

nder consolidation: The soil which is not fully consolidated under the existing

overburden pressure is called an under-consolidated soil.

;y using the over consolidation ratio: 6#5 /c9/, where:

/ is existing overburden pressure / .\ \: is sampling depth

?e can determine the consolidation state of a given soil mass.

6bviously when 6#5 R %: The soil is over consolidated

6#T %: The soil is normally consolidated

6#5 K %: The soil is under consolidation

&.3.5 Coeicient o cono)i*(tion HC7

The term coefficient of consolidation #v, is adopted to indicated the combined effect of

permeability and compressibility of soil on the rate of volume change. =t is expressed as follows:

#v (-H

=f the average values of !, a, e They change with increase of pressure /. Laboratory experiments

have already demonstrated that: on the e-log / curve, the pressure /c corresponding to the

beginning of linear portion is value of the maximum stress overburden pressure to which a soil has

been subBected and under which it got consolidated in its stress history. This pressure is !nown as:

/re- consolidation pressure.

Determination of /re-consolidation pressure

Lecture notes by 8ng. 8sdras @48\11S6, ;"c #88T, +"c 48+. %H


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The approximate value of the preconsolidation pressure /c may be determined by the following

method of #asagrande %(3':

%. The point of maximum curvature + on the curved portion of the e-log / plot is located.

2. 1 hori$ontal line +" is drawn through +:

3. 1 tangent +T to the curved portion is drawn through +0

C. The angle "T+ is bisected, +; being the bisector.

). The straight portion D# of the plot is extended bac!ward to meet +; in 8 point.

'. The pressure corresponding to the point 8 is the most probable past maximum stress or the

preconsolidation pressure /c are !nown.

6ther method of determining #v is given by the relationship between elapsed time t and deal

readings sample thic!ness which are consolidated data obtained in Laboratory during

consolidation test.

6ut of many methods available, two maBor methods are hereby described:%. "uare root of time fitting or Tylor method0

2. Logarithm of time fitting or casagrande method.

S('e 'oot o ti9e ittin+:

%. /lot the curve between suare root of time and compression dial reading of specimen.

2. The straight portion of curve 5----- is extended bac! to form the line 1 which meets the

ordinate at reading 5c which is the corrected reading.3.


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C. =ntersection of line ; and curve gives point / corresponding to (* > whose dial reading

and time are respectively 5(and t(.

). ence

C7 T7/&*2 t&

?here d- is average drainage path

Tv Q #onsolidation time factor from the table

> Q Degree of consolidation

Lo+('it-9 o ti9e ittin+:

%. /lot the curve between log t and compression dial reading.

2. 1 tangent at the point of inflection and the asymptote of lower portion of the curve intersect

at point / corresponding to %*>.

3. 1 point 1 corresponding to t % minute and point ; corresponding to t minute are

mar!ed located on the curve.

C. 1n hori$ontal line is drawn at a vertical height \ above point ;, where \ is drawn at a

vertical distance between 1 and ;. The ordinate corresponding to this hori$ontal line is

corrected reading 5c which correspond to >

). The dial reading 5% corresponding to >% %* is given by ordinate of point / . 1fter

locating 5c and 5%the dial ready 5)and hence t)corresponding to > )* can be found

out from the plot.

ence C7 T7/5*2 t5 (-%

soil mechanics module, ced 201

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